Semiconductors such as AlN, GaAs, GaN, InP, Si and SiC may be formed by a vapor deposition method. Examples of the vapor deposition method to be used include a chemical vapor deposition method (CVD method) and a molecular beam epitaxy method (MBE method). In such a vapor deposition method, a substrate is placed in an evacuated chamber and a film is formed such that raw material molecules are supplied in the form of a raw material gas or other appropriate form onto the substrate to deposit a crystal layer on the surface of the substrate.
In this kind of vapor deposition method, the temperature of the substrate in the chamber may have to be accurately controlled in order to densely form an impurity-free semiconductor crystal layer at a constant deposition rate in a reproducible manner. This will be described in detail. Material of the substrate and material of the film to be grown on the substrate surface may usually be different and an optimum temperature for the vapor deposition also differs depending on the composition of the film to be formed. It is therefore necessary to control the heater for heating the substrate to vary the substrate temperature in a planned manner during the film formation and finally bring back the substrate temperature to the ordinary temperature from the temperature suitable for the film formation.
However, internal stress may occur in the semiconductor layer and substrate due to the temperature variation of the substrate to cause a warpage of the semiconductor layer, rather than allowing the semiconductor layer to have a flat surface, in accordance with the combination of the materials used for the film formation, or distribution of the coefficient of thermal expansion in the film, or distribution of the thickness of the film formed on the surface of the substrate. If such a warpage occurs in the semiconductor layer and substrate, the semiconductor layer will crack when cooled after being formed and may possibly break. The film forming condition may therefore have to be controlled in real time so that the surface profile of the semiconductor layer remains flal as much as possible. This control may be performed through preparing a material in a gas state other than the raw material such as for the semiconductor layer and mixing them to be supplied onto the substrate, thereby giving stress to generate bending force opposite to the warpage direction.
Patent Literature 1 below discloses a technique for measuring the surface profile of a semiconductor layer. Specifically, a certain point of the surface of a semiconductor layer is irradiated with a single laser beam of which the irradiation direction is fixed, and the reflected beam from that point is detected using a beam position sensor (PSD: Position Sensitive Detector). The detected position (direction) of the reflected beam is used as the basis to calculate the angle of the surface.
In the invention as disclosed in Patent Literature 1, only one point can be irradiated with the laser beam if the semiconductor layer is in a stationary state, because the irradiation direction of the single laser beam is fixed. Accordingly, the semiconductor layer is rotated to move so that a plurality of points on the surface of the semiconductor layer is irradiated with the laser beam, and the angle can thus be detected at each of the plurality of points on the surface of the film.
When the single laser beam with the fixed irradiation direction is used, however, the amount of information is too small to accurately perceive the surface profile of the film. In addition, it is difficult to accurately perceive the curvature and other necessary parameters within a limited region because the laser beam is difficult to be given to a plurality of incident points within such a limited region.
Moreover, when the film such as a semiconductor layer is in a stationary state or rotates around its axis while only one point is irradiated with the laser beam, if the irradiated point by the laser beam is located at the top of a warpage, the reflection effect of the laser beam is the same as that when the irradiated point is located on a flat surface. Thus, the warpage of the film cannot be accurately measured.
If a plurality of laser beams is given to a plurality of incident points on the surface of a film, the amount of information for measuring the surface profile of the film will increase. In this case, however, the number of components of the apparatus is unduly large because a plurality of light-emitting devices has to be used.